Because of the rapid development of the electronic industry in recent years, the demand for high-speed computing performance of electronic equipment and highly dense packing of integrated circuits on electronic blocks points to the trend of development for electronic equipment. An adverse effect of this trend is that the working temperature and heat density among electronic elements in the equipment will rise speedily during operation; consequently, the life and reliability of the electronic elements are reduced.
In electronic equipment, the working temperature of an element (e.g., an IC) itself as well as the working temperature among all the elements are not the same; some parts may produce very high temperatures. To distribute the heat generated in work by electronic elements or electronic equipment, the conventional method is to provide forced ventillation to distribute heat, or in some cases, water cooling devices are also employed in conjunction therewith to help distribute heat. But as mentioned above, there is the requirement for dense packing of electronic elements on boards to make the product more compact; therefore, the conventional method of using fans or water cooling devices to distribute heat is no longer suitable. There is a need to design a new heat pipe which does not occupy much space but can speedily distribute the heat generated in certain parts of the electronic equipment so that each electronic element stays at a relatively uniform working temperature, thus effectively maintaining the life and reliability of the electronic elements.
The technique of using heat pipes to distribute heat has been gradually adopted in certain equipment. But until now, conventional heat pipe structures cannot be directly applied to electronic equipment to solve the problem confronted by the electronic industry in its development. The reasons for this will be discussed hereinbelow.
The first publication of the principles and techniques of heat pipes was at Los Alamos Scientific Laboratory in 1964. As for the theory and practice of heat pipes, the book Heat Pipe Theory and Practice by S. W. Chi, McGraw-Hill, 1986, provides useful information.
Like conductive materials, heat pipes transfer heat from one place to another, but they have better thermal conductivity.
There are many inventions related to heat pipes and which were granted patent in the United States. Some of these are improvements on application techniques of conventional heat pipes, and reference may be made to their background of invention. These U.S. patents are discussed below:
U.S. Pat. No. 4,799,537 to Bryan C. Hoke, Jr. discloses a self-regulating heat pipe.
U.S. Pat. No. 4,941,527 to Toth et al. provides a sealed casing connected to an evaporator and a condenser in a heat pipe, forming a widening vapor flow passage from the evaporator to the condenser.
U.S. Pat. No. 4,995,450 describes a structure with internal spiraled grooves for enhanced thermal conductivity of the working fluids.
U.S. Pat. No. 5,044,426 to Kneidel teaches a heat pipe the interior thereof having a ligament for fixing a restriction member which extends from a noncondensible gas zone to a working fluid zone to reduce the internal cross-sectional area of the heat pipe.
The heat pipe is a hollow enclosed vessel which is made vacuum and then filled with a working fluid. When the heat pipe contacts a heat source, the temperature of the part of the heat pipe that is in contact with the heat source will rise. The absorbed heat will heat the working fluid in the vicinity of the inner wall of the part of the heat pipe in contact with the heat source until the working fluid is evaporated. At this time, the vapor pressure rises and pushes to the other areas of lower pressure, producing a vapor current flow, the vapor is then cooled and condensed to liquid, and by means of capillary structures, the condensed liquid is returned to the heated part of the heat pipe by capillary action. This liquid is again evaporated and the whole cycle is repeated. In this way, heat absorbed from a heat source by a certain part of the heat pipe is speedily distributed to the other parts thereof.
The capillary structures of prior plate type heat pipes include mainly the mesh capillary system and sintered metal layer system, wherein the mesh capillary system is by using metal coils or springs which extend within the heat pipe to secure the mesh tightly to the inner walls of the heat pipe, while in the sintered metal layer capillary system, a layer of metal powder is fixed on the inner walls of the heat pipe and is sintered in shape using a high temperature furnace. These two conventional capillary systems of heat pipes have their respective drawbacks as described below:
1. In adopting the mesh capillary system, metal coils must be used to support the mesh so that it tightly attaches to the inner walls of the heat pipe; this not only increases cost, but the capillary efficiency is also affected by the distance between the strings of the mesh. In fact, the mesh cannot be perfectly and uniformly attached to the inner walls of the heat pipe; the mesh is actually secured tightly to the inner walls in some parts and loosely in certain parts. Therefore, in practical use, the part where the mesh has loose contact with the inner wall, there is a relatively high heat resistance. Besides, this method of forming the mesh capillary structure does not allow the plate cross-section to have a length to width ratio that is too great.
2. In the sintered capillary system, since it is formed by sintering a layer of metal powder on the inner walls of the heat pipe, it is not suitable for use in a heat pipe with flat and wide inner walls. As is well known to those skilled in the art of sintering, it is not an easy job to evenly distribute metal powder grains on each cross section, not to say sintering them into shape.
Therefore, the reasons why conventional heat pipes cannot be directly applied to electronic equipment is because future electronic equipment requires a heat pipe that is thin and, preferably, has no restriction on the width or length. But the above-described conventional heat pipes cannot meet this requirement. Furthermore, a thin heat pipe must have good performance during the manufacturing process because it must be prevented from shrinking when it is made vacuum during the process. Besides, when it is in use and heated, it may not expand and distort in shape when its internal vapor pressure increases.